A new technique that might lead to the
quick, accurate and safe detection of nuclear materials smuggled inside
sea-going cargo containers has been demonstrated by a team of researchers
with Lawrence Berkeley National Laboratory, the University of California
at Berkeley, and the Lawrence Livermore National Laboratory.

At U.S. seaports such as this one
in Miami, nearly 7,000,000 cargo containers are offloaded each year.
A container may hold about 20 metric tons of goods and represents
a prime target of opportunity for smugglers.

Using Berkeley Lab's 88-Inch Cyclotron, the researchers showed that irradiating
a cargo container's contents with a beam of neutrons, then measuring the
emission of high-energy gamma rays, provides a unique identifying signature
of plutonium or highly enriched uranium that can be used for weapons of
mass destruction. A detection system based on this technology has the
potential to screen every cargo container entering a United States seaport.

"Our method is based on the fact that neutron-induced fission of
special nuclear materials is followed by beta decays of short-lived fission
fragments during which large numbers of high-energy gamma rays [above
3 million electron volts] are emitted," the authors write in a paper
that will appear in the journal Nuclear Instruments and Methods in
Physics Research.

"These gamma rays have energies above those of natural background,
are emitted with significantly greater intensity than beta-delayed neutrons,
have much higher probabilities of escaping hydrogenous cargo loadings
than neutrons, and their energy spectra and time dependencies provide
a unique signature of special nuclear materials," the authors write.

Homeland security experts say that terrorist threats are most likely
to enter our nation by way of the sea, in one of the nearly seven million
cargo containers offloaded at U.S. ports every year. These tractor-trailer-sized,
steel-walled boxes are typically sealed shut in foreign ports and not
opened until delivered by trucks to points all across the country.

Despite heightened security concerns following the September 11 terrorist
attacks, less than two percent of these containers are ever opened and
inspected at U.S. seaports, according to the U.S. Customs and Border Protection
agency. It is widely agreed that improved technologies for the nonintrusive
inspection of cargo containers are sorely needed. Of greatest concern
is the search for the so-called special nuclear materials, such as plutonium
or uranium 235. Although only mildly radioactive, special nuclear materials
in concentrated form can serve as the primary ingredients of nuclear explosives.

"It only takes about 10 kilograms of special nuclear materials to
make an atomic bomb, which is about enough material to stuff inside a
baseball," says Norman. "You have to be able to find this when
it's hidden in a container that might be holding nearly 20 metric tons
of cargo."

Whereas x-ray imaging can reveal the presence of an object inside a container
that does not conform to a declared manifest, Norman and Prussin say that
detecting the emission of high-energy gamma rays gives "a definitive
yes or no" as to whether the smuggled object contains special nuclear
materials.

Norman and Prussin got their idea for using gamma-ray emissions to detect
the presence of special nuclear materials while serving as consultants
on a project at Livermore led by Dennis Slaughter, technical director
of the 100 MeV Electron-Positron Linac Facility. Livermore researchers
were exploring the possibility of bombarding a cargo container with energized
neutrons that would trigger a brief fission reaction in plutonium and
enriched uranium, while not significantly affecting elements such as hydrogen,
carbon, oxygen, or calcium, which make up food products and other materials
commonly shipped by sea.

Among the emission products from this fission reaction are "delayed
neutrons"  those slowly emitted following an initial burst
of released neutrons  which the Livermore researchers were looking
to measure. Norman and Prussin were concerned that a heavy presence of
hydrogen in a cargo container would present a severe roadblock to measuring
neutron emissions, as hydrogen readily absorbs delayed neutrons.

"Hydrogen is all over the place in containers," says Prussin.
"We import fruits and vegetables, filled with water. Computers? They're
made of plastic, predominantly composed of carbon and hydrogen. However,
there are other emission products less prone to absorption in hydrogenous
materials. Our thoughts turned to high-energy gamma rays, which can penetrate
through hydrogenous materials a heck of a lot better than delayed neutrons
can."

Calculations showed that the relative intensity of delayed high-energy
gamma rays is approximately 10 times greater than delayed neutrons and
that gamma rays would get through hydrogenous materials anywhere from
100 to 1,000 times more easily. This means that under some circumstances,
gamma rays would be as much as 10,000 times more sensitive a means of
detecting special nuclear materials than delayed neutrons.

As a proof-of-principle experiment, Norman and Prussin and their Berkeley
and Livermore collaborators generated 8-million-electron-volt beams of
neutrons at the 88-Inch Cyclotron, which they moderated and then used
to irradiate sample targets of plutonium 239 and uranium 235, plus a variety
of materials that might be found in a typical cargo container including
wood, polyethylene, aluminum, and steel. They then measured gamma-ray
emissions, using both a high-resolution germanium detector and a low-resolution
plastic scintillator.

"We found that gamma rays were emitted in a wedge-shaped energy
spectrum that is well above any normal background that may be present
during the screening process," Norman says. "We also saw a characteristic
half-life decay of approximately 25 seconds. All other materials showed
much longer decay times."

Based on their tests at the 88-Inch Cyclotron, the collaboration concluded
that with this technology an entire cargo container might be screened
for the presence of special nuclear materials in about one minute. Using
relatively inexpensive plastic scintillators to detect the gamma rays
is every bit as effective as the more costly germanium detectors, which
helps hold down the cost of a detection system and also speeds up the
process. The next step calls for experiments using a full-size container
that's packed with mock cargo and a hidden sample of special nuclear material.
These experiments are already being planned to take place at Livermore
Lab's test facilities.

Norman says a full-scale system for use at a major seaport could be built
at an economically practical cost. While tests at Livermore are needed
to determine whether the technology will be effective for use in seaports,
Prussin is optimistic. He says, "I think this method has a much higher
probability of success than anything that's been suggested so far."